“Race is the Rubicon we have never crossed in this country.” That’s David Billings’ thesis in his provocative new historical memoir, “Deep Denial: The Persistence of White Supremacy in the United States History and Life.” It documents the 400-year racialization of the United States and how people of European descent came to be called “White.” Billings tells us why, despite the Civil Rights Movement and an African-American president, we remain, in his words, “a nation hard-wired by race.”